There is ongoing interest in expanding and improving the measurement of semiconductor wafers. A number of optical metrology tools have been developed for non-destructively evaluating the characteristics of thin films formed on semiconductors during the fabrication process. More recently, optical metrology systems have been proposed for analyzing the geometry of small periodic structures (critical dimensions) on semiconductors.
Typical optical tools include reflectometry (both single wavelength and spectroscopic) and ellipsometry (again, both single wavelength and spectroscopic.) In some metrology tools, these various techniques are combined. See for example U.S. Pat. Nos. 6,278,519 and 5,608,526, the disclosures of which are incorporated herein by reference.
Other metrology tools have been developed which rely on measurements at multiple angles of incidence (both single wavelength and spectroscopic). One class of such systems have been commercialized by the Assignee herein are capable of deriving information about multiple angles of incidence simultaneously. In these systems, a strong lens (high numerical aperture) is used to focus a probe beam of light onto the sample in a manner to create a spread of angles of incidence. An array detector is used to measure the reflected rays of the probe beam as a function of the position within the probe beam. The position of the rays within the probe beam corresponds to specific angles of incidence on the sample. Theses systems are disclosed in U.S. Pat. Nos. 4,99,014 and 5,042, 951, incorporated herein by reference. U.S. Pat. No. 4,99,014 related to reflectometry while U.S. Pat. No. 5,042, 951 relates to ellipsometry. (See also, U.S. Pat. No. 5,166,752, also incorporated herein by reference).
In a variant on this system, U.S. Pat. No. 5,181,080 (incorporated by reference), discloses a system in which a quad-cell detector (FIG. 1) is used to measure the reflected probe beam. Each quadrant 1-4 of the detector measures an integration of all of the angles of incidence falling on the sample. By subtracting the sums of opposite quadrants, ellipsometric information can be obtained. As described in the latter patent, the information derived from the analysis corresponds to the ellipsometric parameter delta xcex4 which is very sensitive to the thickness of very thin films on a sample.
The concepts of the latter patents were expanded to provide spectroscopic measurements as described in U.S. Pat. No. 5,412,473, also incorporated herein by reference. In this patent, the system was modified to include a white light source. In one approach, a color filter wheel was used to sequentially obtain multiple wavelength information. In another approach, a filter in the form of a rectangular aperture was used to select a portion of the reflected beam. This portion was then angularly dispersed onto an array with each row providing different wavelength information and each column containing the various angle of incidence information.
U.S. Pat. No. 5,569,411, also incorporated by reference, disclosed a preferred approach for obtaining spectroscopic information for an integrated multiple angle of incidence system of the type described in U.S. Pat. No. 5,181,080 discussed above. In this approach, a filter was provided that transmitted light along one axis and blocked light along an orthogonal axis. The transmitted light was angularly dispersed and measured to provide spectroscopic information along one axis of the probe beam. The filter was then rotated by ninety degrees to obtain measurements along the remaining axis. Various modifications of this approach were discussed, including splitting the beam and using two identical filters disposed orthogonal to each other to obtain both measurements simultaneously. (See also xe2x80x9cCharacterization of titanium nitride (TiN) films on various substrates using spectrophotometry, beam profile reflectometry, beam profile ellipsometry and spectroscopic beam profile ellipsometry,xe2x80x9d Leng, et. al., Thin Solid Films, Volume 313-314, 1998, pages 309 to 313.)
The integrated multiple angle ellipsometric measurement system described in U.S. Pat. No. 5,181,080, cited above has been successfully commercialized and is incorporated into the Opti-Product sold by the Assignee herein. The technology is marketed under the trademark Beam Profile Ellipsometry. (See U.S. Pat. No. 6,278,519 cited above.) As described in the ""080 patent, the four segments of the quad cell detector can be summed to provide information about the total reflected power of the probe beam. In addition, the sum of the output of the quadrants along one axis can be subtracted from the sum of the outputs of the remaining two quadrants to provide a result which is corresponds to the ellipsometric parameter xcex4.
This arrangement provides valuable information that can be used to determine the thickness of thin films. However, the limited information from this type of detection cannot typically be used to derive both of the ellipsometric parameters, xcexa8 and xcex4. U.S. Pat. No. 5,586,411, discloses that it would be possible to derive such information if one of polarizers were rotated and multiple measurements taken. As noted therein at column 12, line 48, if enough measurements are taken, a Fourier analysis can be performed on the data allowing the parameters of xcexa8 and xcex4 to be extracted.
When designing commercial inspection systems, it is often desirable to minimize the number of moving parts. For example, moving parts often create particulates that can contaminate the wafer. To the extent parts must be moved, the motion systems must have high precision. Further, movements of parts that are specifically designed to modify optical properties, such as retarders or polarizers can effect how the system transmits and detects light.
Therefore, it is an object of the present invention to enhance the operation of an integrated, simultaneous multiple angle ellipsometric system without the drawbacks of the prior approaches. In particular, the subject invention is intended to permit the derivation of additional ellipsometric information, including both xcex4 and xcexa8. In one class of embodiments, this additional information is derived in a system with an improved detector arrangement without the need for moving parts. In another class of embodiments, the rotating element is limited to the detector which does not effect the polarization or retardation of the light.
In a first embodiment, a narrowband light source such as a laser is used to generate a probe beam. The polarized beam is focused onto the sample in a manner to create a large spread of angles of incidence. The probe beam light is passed through a quarter waveplate for retarding the phase of one of the polarization states of the beam with respect to the other polarization state of the beam. A polarizer is provided for creating interference between the two polarization states in the reflected probe beam.
In accordance with the subject invention, a detector is provided with eight segments radially arranged around a center axis. In one embodiment, eight pie shaped sections are provided in a configuration which is essentially a quad cell with each quadrant further divided in half. In another embodiment, the eight segments are arranged in an annular ring.
In either case, the output of the segments lying substantially along one radial axis is subtracted from the output of the segments lying substantially along an orthogonal radial axis. In order to gain additional information, the output of the sectors lying along a third radial axis located midway between the first two orthogonal axes (i.e. at 45 degrees) is subtracted from the sectors lying along a fourth radial axis, perpendicular to the third axis. This extra information obtained corresponds to an orientation essentially shifted by 45 degrees from the first measurements. When all the measurements are combined, the sample may be more accurately evaluated. This extra information can be supplied to conventional fitting algorithms to evaluate characteristics including thin film thickness, index of refraction and extinction coefficient. In addition, geometrical parameters of structures formed on semiconductors such as line width, spacing, and side wall angle and shape can also be evaluated. These calculations can be made using the measurements directly. Alternatively, the measurements can be used to derive the ellipsometric parameters xcexa8 and xcex94 which are then used to evaluate the sample.
The use of an eight segment detector allows the extra measurement to be obtained simply by summing and subtracting outputs of the sectors in the processor. Other arrangements can be used to provide equivalent results. For example, the reflected beam could be split into two parts and the two quadrant detectors use, one offset by 45 degrees from the other. Alternatively, a single rotatable quadrant detector could be used. After the first measurement is made, the detector could be rotated by 45 degrees and a second measurement could be made. In a preferred embodiment, the output of all the segments is summed to provide a measure of the full power of the reflected beam.
It may also be possible to rotate the either the polarizer or the retarder to achieve a similar result.
The concept can also be extended to the use of a two dimensional detector array. Using a processor, the elements on the array can be computationally mapped to the eight segment and the analysis can be made as described above. This approach can be particularly useful for measurement of critical dimensions, where the orientation of the sample structure and orientation of the probing radiation is significant and possibly difficult to control.
The subject invention can also be extended to spectroscopic measurements. In this case, a white light source would typically be used to generate a polychromatic probe beam. The probe beam could be passed through a color filter or monochrometer which sequentially transmits narrow bands of wavelengths. The filter or monochrometer would preferably be located before the sample.
If simultaneous multiple wavelength information is desired, it is believed that the approach described in U.S. Pat. No. 5,596,411, which included a rotating quadrant filter, grating and array detector would be more suitable than the approach described herein.
Further objects of the subject invention can be understood with reference to the following detailed description, taken in conjunction with the drawings in which: